Just thought I'd post some helpful tips on how I typically do my DOFFing. Results may vary, especially after the new system is implemented in Season 9.5 (particularly with that heinously bad interface nearly universally hated by Tribble testers) but I've found it to be pretty good for farming.
Specifically, one of my main goals is to visit several cluster/nebulae (which will technically be rendered obsolete in 9.5, but their assignments should still exist) in order to queue up some extremely good assignments for CXP. First though, there are some things you kind of need to maximize the effectiveness of this type of run. Most important are the DOFFs themselves, several types of which can be farmed in perpetuity once you've completed the colonial chains in specific clusters. The "Support Colonization Efforts in X" assignment is repeatable, and critical success nets you a unique but not really unique DOFF.
Astrometrics - Hromi (Pi Canis) - Good for Establish Listening Post.
Technician - B'tran (Gamma Orionis) - Good for Establish Listening Post. Also good if you run an Aux2Bat setup.
Doctor - Delta Volanis (Sirius) - Better for Feds, a two-trait DOFF for farming Support assignments. The more of her you have, the more Supports you can run simultaneously, and best of all, DV is practically right next door to Sol.
Shield Distribution - Rolor (Beta Ursae) - Good for Establish Military Base. Also great for KDFs with all their freighter disabling and raiding assignments. Also a prime two-trait Support for KDF.
Projectile Weapons - Khazan (Alpha Centauri) - Ditto the above.
Assault Squad - Betreka (Alpha Trianguli) - Good for Establish Military Base. Good for KDF assault assignments, and even better for the Suppress Gorn assignments.
Tractor Beam - Arucanis (Regulus) - Mainly for KDFs, good for disable freighter assignments.
As noted in their individual sections, you'll want tons of the Fed doctors or KDF shield dist/projectile weapon DOFFs, as you can then keep using those ones to farm more of all the rest.
Some of the most valuable assignment for CXP are as follows:
Establish Listening Post in X - With all purple DOFFs, this assignment will get you over 2.5k CXP each in engineering, espionage and colonial. Even a regular success gives a respectable amount. If in a cluster, always check your Ops department head in case you don't see it in your regular list.
Establish Military Base in X - All purples gives you over 4k each in engineering and colonial on crit. Check your Ops DH.
Suppress Gorn Uprising on X - Only available for KDF, mostly occur in Eta Eridani. Always check your tactical department head as well, as additional instances may occur there. All purples will give you over 9k in military on crit. Check your Tactical DH.
Provide Shore Leave Services - The motherlode of CXP. Occurs only in the Solenae Dyson Sphere. Has a cooldown of about 24 hours from completion. An all purple crit is over 24k to development.
Valuable contraband farming assignments:
Inspect Civilian Freighter - Can get you 5 contraband on crit. Available to both sides. The security DOFF from Tong is perfect for Feds, but oddly has a wrong trait for KDF. KDF can get a purple security officer from Instigate Defection that is perfect though. There's also Confiscate Contraband, but that one's crap. Even a crit typically gets you 2 at most, with some useless junk thrown in.
Disable X Freighter - KDF only. Contraband varies depending on species. Most will have just 1 on regular success. Ferengi have 5 on crit, while most others have 3, but I think Vulcans have none.
Raid on X - KDF only. Can occur almost anywhere. 5 contraband on crit.
Strike Against Fugitive Support Network - Requires completion of Facility 4028 assignment chain. 5 contraband and a blue or purple Dominion fugitive prisoner DOFF on crit. Can exchange the Dominion fugitives for a usable DOFF with the Exchange Dominion Prisoner assignment that occurs in Beta Ursae, Alpha Triangulis, Zeta Andromedae sector blocks.
Now for a good path through space. This flight plan doesn't account for any cooldown reductions you might have for transwarp, so it tries to make the fewest transwarps as possible (namely, one at most in each direction).
For Federation characters: Starting from ESD, I typically pick up a contraband turnover assignment, beam down to SFA, do the lore/history daily, then beam directly to the Sirius sector block. Picking up anything you find valuable along the way through this whole path, head south to Eta Eridani. In EE, stop over at the fleet starbase if you have any CXP trades to do, as well as if you do the dilithium mine dailies (our Fed fleet has direct transwarp, making it easier). Exit the starbase, and fly east to Omega Leonis. In OL, stop by as many clusters as you feel like, picking up any ELP or EMB as you might find. While not as important as the ones listed above, security officers from Tong are useful for the investigate civilian freighter and confiscate contraband assignments. After you've finished up in OL, head north to Pi Canis. Fly toward Tau Dewa, but make a stop in Hromi if you have a lot of slots left. In TD, fly straight for the sphere. Pick up your PSLS. That's where I usually stop for the day on that character.
The next day, I make the return trip. From TD, I instead go to Regulus, where I make a stop in Arucanis. Leaving there, I head back to EE again, and make a stop at the fleet starbase for CXP trades and mining. From there, I head back north to Sirius, making a stop at the Gamma Orionis transwarp gate. After entering GO, I fly straight for B'tran. After picking up whatever is good there, I'll use transwarp to ESD (pop back out to Sirius if you want to make a quick trip to DV), where I beam down to SFA for dailies, then back up to ESD to trade contraband, and thus repeats the cycle.
For KDF characters: Starting from First City, do your contraband trades and also turn over prisoners if you really have to (though you might save them for forced labor camps in the clusters as your success rate will be higher). Beam down to KDF Academy for dailies. Beam directly back up to OL and head west. I don't visit the clusters on the way out as it's a bit out of the way. In EE, visit the fleet starbase. (Our fleet is sadly behind, so no direct transwarp to mine or good mining dailies...yet.) Exit starbase, head north to Pi Canis, stopping in Hromi. Head north to TD, pick up PSLS at there Sphere. Head west to Regulus, then to Sirius. In Sirius, fly to the GO transwarp gate. The KDF is a bit bugged, so you'll have to stop in the Battlegroup Omega map before entering GO, but do so, then go straight for B'tran. From B'tran, transwarp to Beta Ursae. Go north to Rolor. If at this point you still have slots remaining, you can exit Rolor, fly south to Alpha Trianguli, then into Betreka. There you can break for the day.
The return trip the next day is basically a mirror trip, unlike the Fed side. However, flying through Regulus you might stop in Arucanis. There should be no need to stop in the Sphere unless you forgot to pick up PSLS the previous day. Once you get back to OL, then you can just hit each cluster as you like. Leave a slot open for contraband trade if you have enough.
Now these are just suggested routes. Obviously the Fed one doesn't take you into the Cardie triangle (BU/AT/ZA) so you can work that in as you like. If you like to do lots of STFs and the like, you can always just have those queued as you fly around.